This invention relates to a method of increasing the displacement of a motorcycle engine by increasing the bore diameter of its cylinder block and the diameter of its piston, and a piston and a cylinder block used in the engine of which the displacement has been increased according to the method of the present invention.
Ordinarily, to increase the displacement of a motorcycle engine, its piston is replaced with one having a larger diameter, and its cylinder block is replaced with one having a greater bore diameter or is dismounted from the engine and its bore diameter is increased by boring so that the piston with a larger diameter can be received therein.
As shown in FIGS. 3A and 3B, the cylinder bore 12 extends through the cylinder block 2 from its end facing a crankcase 1 to its end facing a cylinder head 7. The cylinder block 2 has its bottom end 2a inserted in and fixed to the crankcase 1.
A cylinder sleeve may be pressed in the cylinder bore 12 to extend into the crankcase 1.
The piston 10 has at its lower portion a lateral boss 10b, and is connected to the top of a connecting rod 4 by means of a piston pin 15 inserted through the boss 10b and a pin hole 4a (see FIG. 2, which itself represents an embodiment of the present invention) formed in the top end of connecting rod 4 so as to align with the boss 10b. 
The connecting rod 4 is connected at its bottom end to a crank pin 17 of a crankshaft 3 so that the piston 10 reciprocates between a top dead center U shown by phantom line in FIGS. 3A and 3B and a bottom dead center D shown by solid line in FIGS. 3A and 3B.
In order to increase the engine displacement, the piston is replaced with one with a larger diameter, and the cylinder block is replaced with one having a greater cylinder bore diameter or its bore diameter is increased by boring so as to correspond to the piston with an increased diameter. In this arrangement, because the bottom end of the piston 10 is adapted to be inserted into the crankcase 1 and the bottom end 2a of the cylinder block 2 is inserted in the crankcase 1, as shown in FIG. 5, corresponding to the piston with an increased diameter the diameter of the top opening 1a of the crankcase 1 has to be increased from A to B in FIGS. 4A to 4C.
Thus, it is extremely troublesome and time-consuming and costly to increase the displacement of an engine by increasing the piston diameter and the cylinder bore diameter, because most of the parts of the engine, including the piston 10, cylinder sleeve, connecting rod 4 and crankshaft 3, have to be disassembled.